Self-synchronizing communication system



3,158,864 SEEF-YNHRQNEZENG CQMMEJNHCATHGN SYSTEM Frank W. Lehan, Glendale, Caiih, assinorto Space-General Corporation, Giendaie, Caiii. Filed Dec. 27, 196-0, Ser. No. 78,47@

15 (Ilaims. (U. 343-260) employing these techniques is that in the synchronous system the clock-time base constantly requires synchronization during operation and in the non-synchronous,

system the matched filters require a storage capability commensurate with the complexity and variety of the codes employed. Accordingly, there has been a longfelt need for a technique that wouldmalre it unnecessary to either generate or store reference codes for decoding purposes. More particularly, there has been a need for a technique in which the message transmission itself would provide the synchronization required-for accurate decoding. i

The coding and decoding concept of the present invention eliminates the need for receiver synchronization from a stable clock-time base, reference codes in a matched United States Patent 0 filter network, or other methods presently in use. According to the basic concept of the present invention, an incoming signal is decoded by convergence on synchronization rather than, for example, by searching for a correlation peak. Stated ditferentlyQit' is the concept of the present invention that, through the use of feedback principles, the incoming signal itself shall provide the reference signals required for both synchronization and decoding. Thus, systems e mbodying the principles of the present invention are self-synchronizing in nature and when a system receiver is initially out of lock, error signals are generated that force the receiver to converge rapidly to a stable locked condition.

In one embodiment of the invention, the system transmitter includes a number of oscillators whose output signals are generated at respectively different frequencies or,

" if the signals are non-sinusoidal, at respectively different pulse ire'petition frequencies. A multiplier in thetransmitterthencombines the signals out of the oscillators by multiplying them together, the signal thereby formed being a complex pseudo-random type of waveform that spreads the transmitted signal over a large part of the frequency spectrum. Thus, if the output signals or" the ini and S v the multiplier output waveform is the product of S S dividu'al oscillators are indicated as S S and S In the'receiver, as many different channels'are provided as' there iarejoscillators infthe translmi tter, each channel being .associated}with one of the oscillators and designedtoreproduce the signal generated by that oscillators Thus, yway of example, the first channel reproducessig'nal Si, thesecond channel reproduces channel S ,'and 'so on, with 'the nth channel re producing signal S,,..,- Each. of the referred-to channels I includes a balanced modulator and thetransmittcd wayej ff orm, uponreceipt,iszisimultaneously applied to thefmodm lat ors in all the channels. A number of locally. produced I i -reference signals arerespectively applied to these balanced modulators at the same time, each reference signal containing all the signal components transmitted except the one associated with the channel to which the reference signal is applied. The action of each balanced modulator due to its inherent operation as a multiplier is such as to remove all the signal components from the output except the signal component associated with that channel. Thus, for sake of clarity, if the transmitted waveform contains signal components S through S and the locally produced reference signal applied to the balanced modulator in the first channel contains signal components S through S,,, then the modulator output is signal S In a similar manner, outputs S through S are respectively produced by the balanced modulators in the other channels. These modulator outputs are filtered and limited to reduce the noise components therein and then multiplied together again to form a new complex signal which is a duplicate of the transmitted Waveform but relatively noise-free. This newly formed complex signal containing signal components S through 5 is then respectively It is thus seen that a new technique is being employed to provide synchronization for. decoding purposes and' that this technique eliminates any necessity for a clock base or for storage of reference codes. It can also be seen that a system based on the present invention can be expanded to include a large number of frequencies and, hence, very complex electrical waveforms that would 00- cupy a large part of the frequency spectrum can be generated by the system. This is a particularly desirable feature from the standpoint of providing jam-resistant equip ment. More specifically, the self-synchronizing coding technique of the present invention is applicable to and may be used with great advantage in any system requiring a secure communication link since such a technique would be less susceptible to jamming than one using a synchronized time base arrangement.

, It is, therefore, an object of the present invention to provide communications apparatus that is self-synchroa communications system that is extremely difiicult to .jam.

It is a further object of the present invention to provide a communications system wherein multiple feedback is employed for synchronization and decoding purposes. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will bebetter understood from the following description considered in connection with the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be especially understood, however, that the drawings'are for the purpose of illustration-and description only and are not intended as a definition of the limits of the invention.

FTGURES l(a)j l(b.)jar 'eybloclr diagrams; that r'cgj spectivelyillustrate -the transmitter and receiver portionsf of a communications systenrv according to the present invention, the system shown having minimal capacity;

, FIGURE is aflowlchart showing the different signal i waveforms'nevelopedin thecommunications*systeniof FIGS. 1(a) and 1(b ;i.;

FIGURESQQQ andBlb) are block diagrams thatresnaassa spectively illustrate the transmitter and receiver portions of a communications system like the one shown in FlGS. 1(a) and 1(b) but of greatly increased capacity; and

FIGURE 4 is a block diagram of a portion of the receiver shown in FIG. 3(1)) and illustrates one way in which that receiver may be modified.

Referring now to the drawings and in particular to FIGS. 1(a) and 1(b) therein, the transmitter of FIG. 1(a) is shown to include a pair of oscillators Ill and 11 connected to separate inputs of a multiplier 12. Transmitter output apparatus 13 is coupled between multiplier 12 and a transmitter antenna 14. With respect to oscillators Ill and 11, these oscillators may be designed to generate any one of a number of dilferent kinds of oscillations, such as ramp voltages, square waves, sinusoidal voltages, sawtooths, etc. Howeverin order to more clearly describe the invention in the discussion that follows, oscillators and ill will be considered as having been designed to generate square wave waveforms. Furthermore, the signals out of oscillators 10 and ill are generated at difierent frequencies or pulse repetition rates. Accordingly, the square waves out of oscillator 10 are generated at one rate while the square waves out of oscillator 11 are generated at another rate. As for multiplier 12, this type of apparatus produces an output that is equal to the product of the signals applied to it and is well known in the art. In the matter of transmitter output apparatus 13, this apparatus includes amplifying and modulation circuitry which is also well known in the art and, hence, need not be described in further detail.

Looking now to FIG. 1(b), the receiver is shown to include a receiver antenna 15 coupled to receiver input apparatus 16, which, as may be expected, contains amplifying and demodulation circuitry. The output of receiver apparatus 16 is connected to a pair of channels generally designated 17 and 18, each channel including a first balanced modulator, a narrow band filter, a limiter and a second balanced modulator. In channel. 17, the first balanced modulator is designated 20, the narrow band filter is designated 21, the limiter is designated 22 and the second balanced modulator therein is designated 23. Receiver apparatus 16 is connected to the first of two inputs to balanced modulator 2d; the output from this modulator feeding directly into narrowband filter 21. Limiter 22 is coupled between filter 21 and the first of two inputs to balancedmodulator 23, the output end of modulator 23 being connected back to the second input of balanced modulator 20. It is thus seen that the elements of channel 17 form a feedback loop. Considering channel 18, the first balanced modulator is designated 24, the narrow band filter is designated 25, the limiter is designated 26 and the second balanced modulator is designated 27. As before, receiver input apparatus 16 is connected to the first of two inputs to balanced modulator 2d, the second input to this modulator being taken from the output end of balanced modulator 27. Narrow band filter is connected between balanced modulator 24 and limiter 26, the limiter'output being connected to the first of two inputs tobalanced modulator 27. Here too, the elements of channel 18 form a feedback loop.

The receiver further includes a multiplier 28 like the one to be foundin the transmitter of FIG. 1(a). In addition to being connected to modulators 23 and 27, limiters 22" and 26 are separately connected to multiplier 23 and also connected to a pair of output terminals 361 a and Ellb, as-shown-in thefigure. Terminals 30a and 39b constitute the output for the receiver and it is at theseterminals that the signals originally generated'by oscillators 1t) and Ill in the transmitter are finallyobtained. The receiver circult is completed by connecting the output end of multiplier 21 directly to' the second inputof both modulators 23 and 2'7. e

Considering now the operation, oscillators 1d and 111 in FIG. 1(a) generate square wave or binary signals which alternate between plus 1 and minus 1 values. Waveforms representing these signals are shown in FIG. 2 and it will be noticed from them that the signals generated by the oscillators are generated at different rates. The Waveforms are respectively designated S and S waveform S being generated by oscillator it and waveform S being generated by oscillator 11. The signals out of the oscillators are applied simultaneously to multiplier 12 whose output, as was previously mentioned, is the product of the signals applied to it. It will be recognized that since the signal generated by each oscillator alternates between plus 1 and minus 1 values, the product signal out of multiplier 12 will also alternate between plus 1 and minus 1 values. The signal or waveform produced by the multiplier is also presented in FIG. 2 and is designated S 5 therein. The particular configuration of waveform S 8 is dependent upon the frequencies of waveforms S and S and if these frequencies are all related by prime numbers and are phase coherent with each other, then a pseudo-noise sequence is generated similar to that generated by a linear shift register pseudo-noise generator. The spectrum of such a signal and, hence, the spectrum of signal S 5 occupies a wide band of frequencies dependent upon the components that are present.

Multiplier signal S 8 is applied to transmitter output apparatus 13 wherein it is amplified and then used to modulate one or more carrier signals for purposes of transmission. The modulated carrier is then power amplified and finally applied to antenna 14 for radiation into space. The modulation may take the form of phase modulation, frequency modulation, amplitude modulation or any other form of modulation that may be suitable and available. Byway of example, if frequency modulation were being employed, then the carrier signal would be transmitted at one frequency to represent plus 1 values of signal S 8 and at a second frequency to represent minus 1 values. In a similar manner, the carrier signal would be phased differently if phase modulation were being employed.

At the receiver site, the transmitted signal is intercepted by antenna 15 and passed on to receiver input apparatus ldwherein the signal thusly obtained is both demodulated and amplified. Consequently, the signal produced by receiver apparatus 16 and applied to both balanced modulators 2d and is very much the same as signal S 5 shown in FIG. 2, except that a significant amount of noise is now a part of the signal. While signal S 8 is applied to the first inputs of modulators 2i and 24, signal S is applied to the second input of modulator 2d and signal S is applied to the second input of modulator 24. In accordancewith the well known operation of these modulators, the signal out of modulator 20 and applied to narrow band filter 21 is square wave signal S whereas the signal out of modulator 24 and applied to narrow band filter 25 is square wave signal S The output of the modulators 20 and 24 also includes residual transmission channel noise superimposed upon the signals S and S 1 Filter 21 and limiter 22 in channel 17 and filter 25 and limiter 26 in channel 18 are respectively effective in taking out much of the noise from signals S and S with the result that the signals produced at the output terminals of these limiters are relatively noise-free. Square wave oscillation S out of limiter 22 is applied to the first input terminal of balanced modulator 23 and, at the same time, is applied to'multiplier 23. Similarly, square wave oscillation S out of limiter 26 is applied to the first input of balanced modulator 2'7 and is also simultaneously applied to multiplier 28. In multiplier 23, signals S and S are multiplied together so that signal S 5 is reproduced at the output of the multiplier. This signal, namely, signal S 8 is applied tothe second inputs of balanced modulators 23'and'27. In response to signal S and signal S 8 balanced modulator 23 produces signal S; which it applies to the second input ofbalanced modulator iii. In this way, a suitable reference signal lator 2G. 1

is obtained for the second input of balanced modulator 20, that is, a reference signal that can be combined by modulator 20 with the signal out of receiver apparatus 16 to produce signal 8;. In a like manner, balanced modulator 27 acts in response to signal S out of l'miter 26 and signal S 8 from multiplier 28 to produce signal S which it applies to the second input to balanced modulator 24. As previously mentioned, balanced modulator 24 produces square wave oscillation S in response to the two signals applied to it.

Relatively noise-free signals S and S are respectively developed at output terminals 30a and Zlilb and, as heretofore mentioned, these are the signals originally generated by oscillators It and 11 in the transmitter portion of the system. Accordingly, it is seen that messages '15 transmitted by the transmitter are ultimately reproduced in the receiver at output terminals 30a and sea and that i this message reproduction is achieved without the need for receiver synchronization from a stable clock-time base, reference codes stored in a matched filter network, or other methods that may be in use at this time.

The operation of the receiver of FIG. 1(b) may be more clearly understood by considering the fact that the balanced modulators 2t and 23, for example, act as multipliers in the operation of modulation as is well recognized in the literature, for example Electronic Circuits and Tubes, Cruft Laboratory, McGraW-Hill, Inc, 1947, pages 6 60461, table 8.1. This understanding plus the recognition that each channel includes two feedback loops, namely circuits 1'7 and 19, and 18 and 29, operating to produce a clean noise-free signal S from thecomplex received signal S 8 (plus noise).

Considering the case as illustrated in FIG. 2; of binary signals S and S of value either +1 or -l, the input to balance modulator 26: from receiving apparatus 16 is S 8 (plus noise) as illustrated in idealized form less noise in the third curve of FIG. 2. When the second input to the modu1ator 26 is S the productof the two input signals is obtained, namely 8 8 8 or S (S l'nthe case of these binary signals of two values +1 or '1 the quan tity (S is always equal to +1 whereby the'output of modulator 20 with these input signals is always S (plus noise). The latter noise is removed by the filter 21 and limiter 22 and "a clean signal S is applied to the out- 7 put terminal itla'and to balanced modulator 23.

This operation presupposes the existence of signal S at the second input to the modulator 29. This in fact exists since'the modulator 23 operates in thesame manher With the product S 8 applied to one input and the derived sighal'S to the other, the same multiplication takes place and S (S S equals (S0 3 equals S These mutuallydependent feedback loops can be shown to produce the desiredconvergence by considering the extreme case Where S is not present, as occurs in one form of coding. As is well-known in the art, as illustrated by the Craft Laboratory reference supra, row 1, the output messes its of plus 1 and minus 1 and, furthermore, the periods and frequencies of these signals bear no simple relationship to each other. Oscillators 1W to 1%,, are separately connected to a multiplier 101 Whose output, in turn, is connected to transmitter output apparatus lliiZ. Au antenna 1% is connected to the output end of the transiitter apparatus.

The receiver, shown in FIG. 3 ([2), includes an antenna ltl l which is coupled to receiver input apparatus 195, the output end of the receiver apparatus being connected to a plurality of receiver channels equal in number to the number of oscillators in the transmitter. Consequently, there are n receiver channels and these are generally designated res, to ltld Except for the frequencies or pulse repetition. rates involved, channels 106 to 106,, are identical in every respect to channels 17 and 18 in FIG. 1(a) and, hence, need not be further described.

Suflice it to say, therefore, that channels 196 to 106 respectively include first balanced modulators 197 to 107,

narrow band filters 168 to 198, limiters 109 to 169 of a balanced modulator in the presence of one input 5 alone contains only the input frequency. Therefore, S

, (plus noise) from the receiving apparatus 16 applied to balance modulator Ztl produces a noiseS at its output S the input to balanced modulatorfl} from multiplier and a noise-free S at terminal 30a. In the absence of 28 is S and itsoutpl ltls equalsl applied to inodu- Theoutput atthe' terminal 39a therefore isS and by ff analogous,reasoning" the output at terminal '30!) is 0. These extreme conditions illustrate the operation of the receiyerof FIG. 1(b),

' Having-thus described an i embodiment as present r inventionin: its Ibasidform, attention is now directed to FIGS, 3(a) and3(b) wherein is shown the same 'em- 1 i bodiment but veryfgreatlyextended-to provide' -it. with Fol-lowing apparatus M32,

and second balanced modulators 116 to 110 the elements in eachrof these channels being interconnected in exactly the same manner as the elements in previously referred to channels 17 and E8. The output from each of the 11 channels, taken at the output terminals'of limiters E99 to 199 is fed to separate input, terminals of a multiplier 111 whose output terminal is then corn nected to the second inputs of balanced modulators 110 to llll The output terminals of the receiver are nothing more than taps taken off the output lines of limiters 109 to 169,, and they are respectively designated 112 to 112 p v f Considering now the operation, oscillators 19 .9 to

109,, are selectively operable to generate 11 square wave oscillations at it different pulse repetition rates, the message being transmitted at any one time being determined by" the particular combination of oscillators activated or, stated differently, by the particular group of oscillations combined. Thus, for example, it the oscillations capable of being generated by oscillators see, to 1%,, are respectively designated S to then a first message may be.

made up by combining the oscillations out of the first ten oscillators, namely, signals S to S A second message may, by Way of examplebe made up by replacing one of the signal components in the first messa e by a signal not previously used. ltis thus seen that a great many different messages may be transmitted if his a good-sized number. Whatever the message transmitted, the signal components thereof are applied to multiplier 191 wherein they are multiplied together'so that the multiplier produces an output signal that more or less resembles a continuous spectrum. A Waveform that is representative or the type of output signal that maybe produced by multiplier 161 is shown in FIG. 3(a) and is designated S S S S The complex signal produced by the multiplier is applied to transmitter output apparatus 1492 wherein, as previously explained, it" is used to modulate one o-r-"more carrier signals depending upon the type of modulation employed. the message is radiated into space by antenna i 53, 1 7 At the receiver, the message-interceptedby antenna TM is relayedto receiver input apparatus 165 wherein it ,is both amplified and demodulated. The output from I receiverapparatus therefore, aside from the added much greater message,capacity." The transmitterof 'FlGn 3(a) is shown toinclude aplur a'lity of oscillators lue tofltltl T-witheach oscillator enerating "a squareiwave noise, the same as the output from multiplier Tull Thus,

FIG; 3(a), then the very same signal is produced at the output of receiver apparatus 1% and applied to channels" ldd to 1% vThese channels operate in exactly the same way as do channels 1? and 1-3 previously described. By" w'ay'of example, reference is made to channel 165 V wherein the first and second inputs to balanced modulatorllh '7 5 it multiplier Frill has-produced signal 5 3 respectively receive signals 3; and 3 3 5 1 S the essence latter signal coming from multiplier 111. In response to these two signals, balanced modulator 110 applies a signal S S S to the second input to balanced modulator 197 which, because of the signals applied to it, produces signal S as an output signal. Signal S is passed through filter lfifi and limiter 159% for noise elimination and then applied both to multiplier 111 and. to the first input of balanced modulator llll Signal S also appears at output terminal 112;. in the same way, theremaining channels produce signals S S and S at their respective output terminals.

One way in which messages can be transmitted was described above and this way primarily consists in combining different groups of oscillations. Messages can also be transmitted in another way, namely, in a way that involves jittering or varying the pulse repetition rates of the oscillations in a prescribed manner within very narrow limits which, it will be recognized by those skilled in the art, will depend upon the frequency limits of the narrow band filters in the receiver channels. Thus, in addition to being able to form messages by the simple expedient of combining different groups of oscillations, messages may also be formed by slightly varying the pulse repetition rates of the oscillations in any one group. Of course, frequency selection and frequency variation may be combined to make it possible to form still larger numbers and varieties of messages for transmission.

The receivers of FIGS. 1(1)) and 3(1)) may be modified by the insertion of a phase-lock loop in each of the channels thereof, a phase-lock loop aiding in the convergence towards synchronization. The manner in which a receiver channel is modified to include phase-lock loop is shown in FIG. 4 wherein only a single receiver channel is illustrated, namely, channel 1% of FIG. 3(b). One channel only is shown both to expedite the description and for sake of clarity since all channels are modified in exactly the same way. As may be seen from a comparison of channels, channel lfid shown in PEG. 4 differs from channel 196 shown in EEG. 3(1)) in only two respects, namely, the electrical connection between limiter i99 and balanced modulator Hi has been eliminated and a phaselock loop circuit 113 has been interposed between baianced modulator N7 and balanced modulator 110 More specifically, the output end of limiter H39 still connects to multiplier ill and to output terminal 112 but is no longer connected to the first input to balanced modulator 11%. instead, phase-lock loop i13 connects to the first input of balanced modulator Hil In all other respects the interconnections of channel ltl remain the same as they were previously. Phase-loch loop 113 includes a balanced modulator 114- a lo. pass filter 115 and a voltage-controlled oscillator i16 Balanced modulator 114- has two inputs, one input connecting to the out-put end of balanced modulator 1.63 and the other input connecting to the output of voltage-controlled oscil lator H6 Low-pass filter 115 is coupled between the output end of balanced modulator lld and the input end of voltage-controlled oscillator 116 thereby completing the loop. Finally, the output from oscillator llfi is also fed to the first input to balanced modulator 119 Considering the operation of a channel as modified in PEG. 4, the output of balanced modulator 167 is signal S This signal is successively passed through narrowband filter ltlii and limiter lllfi to multiplier Jill as well as to output terminal 112 Signal S out of balanced modulator 107 is also applied to the-first input of balanced modulator lll i a second signal S that is locally generated by voltage-controlled osci lator 115 being applied to the second input of balanced modulator 114 Since these two signals are most probably out of phase with each other initially, an error signal of varying amplitude is developed at the output end of balanced modulator 114 This error signal is'passed to low pass filter 115 which smooths the signal to produce a direct-current error signal which the filter applies to voltage-controlled E; oscillator ill-d In response to the error signal applied to it, the oscillator adjusts the phase of the signal generated by it until it is in phase with the signal out of balanced modulator N7 At this point in time, the error signal is reduced to Zero so that no further adjustment of phase is made by the oscillator.

Properly phased signal 8 out of oscillator 116 is applied to the first input to balanced modulator 119 signal 8 8 8 S being applied to the second input of the modulator by multiplier 111. As a result, the signal out of modulator 119 and applied to the second input to balanced modulator 197 is signal S 8 S Modulator 167 takes the two signals applied to it, namely, signal S S S S applied to its first input terminal and signal S 3 S applied to its second input terminal, and responsively produces signal S at its output end which, as was previously mentioned, it applies to narrow-band filter 103 as well as to balanced modulator 114-; in phase-lock loop 113 The cycle of operation is thus completed.

In the event signal component S is not included in the signal applied to the first input to balanced modulator 107 it will be recognized or may be determined by tracing through the cycle of operation that under these conditions substantially no signal is produced at the output of modulator M7 that is to say, no signal 5 is produced by the modulator. Consequently, no signal S appears at output terminal 112 nor does a signal component S appear in the reference signal produced at the output end of balanced modulator filth and fed back to the second input to balanced modulator 107 It may, therefore,"be further noted that With the receiver channels modified as illustrated and described in connection with FIG. 4, message combinations may be formed by utilizing'the technique of frequency selection or frequency variation or both.

It should finally be noted that still other modifications of the embodiment herein described may be effected without departing from the spirit or scope of the invention. Thus, as a simple example, where noise is not a significant factor for consideration, limiters 169 to Ill? may be eliminated or taken out from the various receiver channels.

Having thus described the invention, what is claimed as new is:

1. Communications apparatus comprising: first transmitter means for generating a pair of signals respectively at first and second frequencies; second transmitter means coupled to said first means for multiplying said signals together to produce a complex output signal; third transmitter means for radiating said output signal through space; first receiver means for intercepting and reproducing said complex output signal; second and third receiver means coupled to said first receiver means to receive said complex output signal, saidsecond receiver means being operable in response to said complex output signal and first reference signal at said first frequency to reproduce the signal at said second frequency and said third receiver means being operable in response to said complex output signal and a second reference signal at said second frequency to reproduce the signal at said first frequency; fourth receiver means coupled to said second and third receiver means for multiplying together the signals at said first and second frequencies to reproduce said complex signal; and fifth and sixth receiver means coupled to said fourth receiver means, and respectively coupled to said second and third receiver means, said fifth receiver means being operable in response t'o-said complex signal and the signal .at said first frequency to produce a first reference signal and said sixth 'receiver means being operable in response to said complex signal and the signal at said second frequency to produce a second reference signaLsaid fifth andv sixth receiver means respectively being additionally coupled to said second and third receiver means for applying said first and second reference signals thereto.

absent from each reference signal being different from 2. Communications apparatus comprising: first transmitter means for generating first and second signals at respectively first and second frequencies; second transmitter means coupled to said first transmitter means for multiplying said first and second signals together to produce a complex output signal; third transmitter means for radiating said complex signal through space; first receiver means for intercepting and reproducing said complex signal; second receiver means coupled to said first receiver means and operable in response to said complex signal and a first reference signal at said second frequency to reproduce said first signal; third receiver means operable in response to said reproduced complex Waveform and said It signal components respectively applied coupled to said first receiver means and operable in response to said complex signal and a second reference signal at said first frequency to reproduce said second signal; and fourth receiver means coupled in a feedback arrangement to said second and to said third receiver means, said fourth means being receptive of said first and second signals and operable in response thereto to produce said first and second reference signals for application to said second and to said third receiver means, respectively. a

-3. A communication system comprising: a transmitter for radiating a complex pseudo-random Waveform constituting the product of n signal components respectively at it different frequencies; first receiver means forlintercepting and reproducing said complex: waveform; n circuits coupled to said first receiver means and operable in response to said complex waveform and n reference signals respectively applied thereto to respectively reproduce said 11 signal components; and second receiver means coupled in a feedback arrangement with each of said u circuits, said second receiver means being receptive of said n signal components and operable in response thereto to produce n reference signals for application to said It circuits, respectively, each of said reference signals having n1 of the n signal components with the missing signal component being different from those-missing fnonr the other reference signals.

4. A communication system comprising: a transmitter for radiating a complex pseudo-random waveform constituting the product of n signal componentsrespectively attvz different frequencies; firstreceiver means for intercepting and reproducing said complex Waveform; nicircuits coupled to said first receiver means to receive said complex waveform, said it circuits being operable in re- :sponse to said complex Waveform and n reference signals respectively applied thereto to respectively reproduce said It signal components; second receiver means coupled to said n circuits for multiplying together said 11 signal components to reproduce said complex Waveform; and n networks coupled to said second receiver meansland respectively coupled to said it circuits, saidn networks being operable in response to said reproduced complex waveform and said it signal components respectively applied thereto to respectively produce nreference signals for application to said n circuits, each reference signal having n.1 of said 11 signal components, .the signal component those absent from the other reference signals.

5. In a communications system wherein a complex pseudo-random waveform constituting the product of n signal components at respectively 11 different frequencies is radiated through space to a receiver site, a receiver: comprising: first receiver meansfor intercepting and re-' producing/[he compieXWaveform; nmmmts coupled 'beingcpcrable initesponse to Said complexwavefo mafld said second signalrcomponent to produce said second reference signaLsaid fifth and sixth means re spectiyely being additionally coupled to. said second and third means. for a plying said first and second reference signals thereto.

said first means to receive the. complex waveform, said 11 circuits being operable in response to the complex wavecomplex waveform; second receiver means coupled to said n circuits for; multiplying together said'n signal components to reproduce the complexrrwaveformgand n netproduce said first and second reference thereto to respectively produce rz reference signals for application to said it circuits, each reference signal having n-l of said 11 signal components, the signal component absent from each reference signal being different from those absent from the other reference signals:

6. In a communications system wherein a complex pseudo-random wave form constituting the product of n signal components at respectively it different frequencies is radiated through space to a receiver site, a receiver comprising: first receiver means forintercepting and reproducing the complex Waveform; n circuits coupled to said first receiver means and operable in response to the complex waveform and 11 reference signals respectively applied thereto to respectively reproduce the in signal components; and second receiver means coupled in a feedback arrangement with each of said 11 circuits, said second receiver means being receptive of the 11 signal components and operable in response thereto to produce n reference signals for application to said' it circuits, respectively, each of said reference signals having n-l of the n signal components with the missing signal component being different from those missing from the other reference signals.

7. In a communication system wherein a complex "Waveform is radiated into space toward a receiver site having first and second signal components at first and first receiver means for intercepting and reproducing the complex signal; second receiver means coupled to said first means and-operable in response to the complex waveform and a first reference signal at the second frequency to reproduce the first signal component; third receiver means coupled to said first means and operable in response to the complex waveform and a second reference signal at the first frequency to reproduce the second signal component; and fourth receiver means coupled in a feedback loop with said second and said third means, said fourth means being receptive of said first and second signal components and operable in response thereto to signals, respectively. w

8. In a communication system wherein a complex waveform .is radiated into space toward a receiver site having rst and second signal components at first and second frequencies, respectively, a receiver comprising: first receiver .means for intercepting and reproducing the com plex waveform; second and third receiver means coupled to said firstmeans to receive said complex waveform, said second means being operable in response to said complex waveform and a first reference signal at the secondfrequency to reproduce the first signal component and said third means beingoperable in response to said complex waveform and a second reference signal at the first. frequency to reproduce thegsecond signal component; fourthreceiver means coupled to said second and fifth means being operable in response to said complex "form and 12 reference signals respectivelyapplied thereto to respectively reproduce the "n signal component's'of the Waveform and said first signal component to produce said first reference signal and saidsixth receiver means,

'' nal components respectively atnidifferent frequencies is produced, receiver apparatus forrepfoducing said sigaj nal components, said receiver apparatus comprising' n circuits receptive of the complex signal and respectively receptive of n reference signals, said it circuits being operable in response to said signals to respectively produce the 11 signal components; means coupled to said 11 circuits for multiplying together said nsignal components to reproduce the complex signal; and n networks coupled to said means and respectively coupled to said n circuits, said It networks being operable in response to said reproduced complex signal and said It signal components respectively applied thereto to respectively produce 11 reference signals for application to said 11 circuits, each reference signal having n1- of said 11 signal components, the signal component absent from each reference signal being that produced by the circuit to which the reference signal is applied.

10. In a communications receiver wherein a complex pseudo-random signal constituting the product of n signal components respectively at n dilferent frequencies is-produced, receiver apparatus for reproducing said n signal components, said receiver apparatus comprising: n first balanced modulators receptive of the complex signal and respectively receptive of 12 reference signals, said n balanced modulators being operable in response to said signals to respectively produce the n signal components; 11 noiserejection circuits respectively coupled to said u first balanced modulators to pass relatively noise-free signal components; a multiplier network coupled to said 11 noise-rejection circuits for multiplying the 12 signal components together to reproduce a relatively noise-free complex signal; and 11 second balanced modulators coupled to said multiplier and respectively coupled tosaid it noise-rejection circuits, said 12 second balanced modulators being operable in response to said reproduced complex signal and the 11 signal components applied thereto to respectively produce n reference signals for application to said 11 first balanced modulators, each reference signal having nl of said n signal components, the signal component absent from each reference signal being that produced by the first balanced modulator to which the reference signal is applied.

11. A communications system comprising: a transmitter including 11 oscillators for generating 11 oscillations respectively at it different frequencies, means for multiplying said It oscillations together to produce a complex pseudo-random signal, and apparatus for radiating said complex signal into space; anda receiver including means for intercepting and reproducing said complex signal, It first balanced modulators receptive of said complex signal and respectively receptive of n reference signals, said 11 balanced modulators being operable in response to said signals to respectively produce said 11 oscillations, n noiserejection circuits respectively coupled to said It first balanced modulators to pass relatively noise-free oscillations, a network coupled to said n'noise-rejector circuits for multiplying together said it oscillations to reproduce a relatively noise-free complex signal, and n second balanced modulators coupled to said network and respectively coupled to said n noise-rejection circuits, said n second balanced modulators being operable. in response to said reproduced complex signal and said 11 oscillations applied thereto to respectively produce 11 reference signals for application to said it first balanced modulators, each reference signal having nl of said n oscillations as components thereof, the oscillation absent from each reference signal being that producedby the firstbalanced modulator to which the reference signal is applied.

,12. In a communications receiver wherein a. complex first circuits receptive oftlie complex signal and respec tively receptive of n reference signals, said It first circuits beingoperable in response tosaid signals to respectively 1 produce the 12 signal components; means coupled to said It 4 pseudo-random signal constituting the product 'of IL sigfirst circuits for multiplying together said n signal components to reproduce the complex signal; 11 phase-locked circuits for newly generating the n signal components, said It phase-locked circuits respectively being coupled to said It first circuits for synchronizing said newly generated signal components with those produced by said 11 first circuits; 11 second circuits coupled to said multiplying means and respectively coupled to said n phase-locked circuits, said n second circuits being operable in response to said newly generated signal components and the reproduced complex signal to respectively produce 12 reference signals, each reference signal having n-1 of said 11 signal components, the signal component absent from each reference signal being that'produced by the first circuit to which the reference signal is applied, said n second circuits respectively being coupled to said it first circuits in order to apply said 11 reference signals thereto.

13. The receiver apparatus defined in claim 12 wherein each of said It phase-locked circuits includes a voltage-controlled oscillator for generating an oscillation at the same frequency as the signal component produced by the associated first circuit; a balanced modulator coupled both to said oscillator and the associated first circuit, said modulator being operable in response to the signals applied thereto to produce a variable voltage indicative of the difference in time of occurrence between said oscillation and the associated signal component; anda filter coupled betweenisaid balanced modulator and said oscillator for smoothing said variable voltage to apply an error signal to said oscillator, said oscillator adjusting the time of occurrence of said oscillation in response to said error signal according to the amplitude and polarity thereof.

14. A communications system comprising: a transmitter including 12 oscillators for generating n oscillations respectively at 11 different frequencies, means for multiplying said 11 oscillations together to produce a complex pseudo-random signal, and apparatus for radiating said complex signal into space; and a receiver including means for intercepting and reproducing said complex signal, It first balanced modulators receptive of said complex signal and respectively receptive of n reference signals, said n circuits being operable in response to said signals to respectively produce said It oscillations, means coupled to said it first modulators for multiplying together It oscillations to reproduce said complex signal; it phase-locked circuits for newly generating the noscillations, said n phase-locked circuits respectively being coupled to said 11 first modulators for synchronizing said newly generated oscillations with those produced by said first modulators; n second balanced modulators coupled to said multiplying means and respectively coupled to said n phase-locked circuits, said It second modulators being operable in response to said newly generated oscillations and the reproduced complex signal to respectively produce 11 reference signals, each reference signal being composed of n-l of said n oscillations, the oscillation absent from each reference signal being that produced by the first balanced modulator to which the reference signal is applied, said n second modulators respectively beingcoupled to said 11 first modulators in order to apply said 11 reference signals thereto.

15. The system defined in claim 14 wherein each of said It phasedocked circuits includes a voltage-controlled oscillator for generating an oscillation at the same frequency as the oscillation produced by the associated first balanced modulator; a third balanced modulator coupled both to said oscillator and the associated first modulator,-

said third modulatorbeing operable in response to the signals applied thereto to produce a variable voltage indicative of the difference in time of occurrence between said oscillations; and 1a filter coupled betw en said third balanced modulator and said oscillator for smoothing said variable voltage to apply an error signal to said oscillator,

said oscillator adjusting the time of occurrence of its 13 14 osciliation in responoe to said error signal according to the OTHER REFERENCES amplitude and polanty thereof Electronic Circuits and Tubes, Cruft Laboratory, Mc-

References Cited by the Examiner Graw-Hill, Inc., 1947, pp. 660-661.

UNITED STATES PATENTS 5 DA ID .REDINBAUGH P" E 2,108,117 2/38 Gardere et a1. 325-329 V G 2,519,223 8/50 Cheek 328-160 XR GEORGE N. WESTBY, Examiner. 

3. A COMMUNICATION SYSTEM COMPRISING: A TRANSMITTER FOR RADIATING A COMPLEX PSEUDO-RANDOM WAVEFORM CONSTITUTING THE PRODUCT OF N SIGNAL COMPONENTS RESPECTIVELY AT N DIFFERENT FREQUENCIES; FIRST RECEIVER MEANS FOR INTERCEPTING AND REPRODUCING SAID COMPLEX WAVEFORM; N CIRCUITS COUPLED TO SAID FIRST RECEIVER MEANS AND OPERABLE IN RESPONSE TO SAID COMPLEX WAVEFORM AND N REFERENCE SIGNALS RESPECTIVELY APPLIED THERETO TO RESPECTIVELY REPRODUCE SAID N SIGNAL COMPONENTS; AND SECOND RECEIVER MEANS COUPLED IN A FEEDBACK ARRANGEMENT WITH EACH OF SAID N CIRCUITS, SAID SECOND RECEIVER MEANS BEING RECEPTIVE OF SAID N SIGNAL COMPONENTS AND OPERABLE IN RESPONSE THERETO TO PRODUCE N REFERENCE SIGNALS FOR APPLICATION TO SAID N CIRCUITS, RESPECTIVELY, EACH OF SAID REFERENCE SIGNALS HAVING N-1 OF THE N SIGNAL COMPONENTS WITH THE MISSING SIGNAL COMPONENT BEING DIFFERENT FROM THOSE MISSING FROM THE OTHER REFERENCE SIGNALS. 